The invention relates generally to rotors and stators for use with progressive cavity pumps or motors. More specifically, to a skinned stator and/or skinned rotor and method of skinning.
Progressive cavity pumps or motors, also referred to as a progressing cavity pumps or motors, typically include a power section 100, as shown in prior art FIG. 1, consisting of a rotor 101 with a profiled helical outer surface 103 disposed within a stator 105 with a profiled helical inner surface 107. Although the stator 105 is shown with a profiled helical outer surface 111, progressive cavity apparatuses are not so limited, for example, the outer surface can be cylindrical if desired. The rotor and stator of a progressive cavity apparatus operate according to the Moineau principle, originally disclosed in U.S. Pat. No. 1,892,217. Preferably, a rotor has one less lobe than a stator.
In use as a pump, relative rotation is provided between the stator and rotor by any means known in the art, and a portion of the profiled helical outer surface of the rotor engages the profiled helical inner surface of the stator to form a sealed chamber or cavity. As the rotor turns eccentrically within the stator, the cavity progresses axially to move any fluid present in the cavity.
In use as a motor, a fluid source is provided to the cavities formed between the rotor and stator. The pressure of the fluid causes the cavity to progress and imparts a relative rotation between the stator and rotor. In this manner fluidic energy can be converted into mechanical energy.
As progressive cavity pumps or motors typically rely on a seal between the stator and rotor surfaces, at least one of the active surfaces preferably includes a resilient or dimensionally forgiving material. An interference fit between the rotor and stator can be achieved if at least one of the rotor or the stator interface surfaces is made of resilient material. A resilient material further allows power section operation with a fluid containing solid particles as the solids can be temporarily embedded in the resilient material at the sealing interface of the active surfaces of a rotor and stator. The resilient material is frequently a layer of elastomer, which can be relatively thin or thick, disposed in the interior surface of the stator. However a layer of resilient material can be disposed on the surface of a rotor. A stator or rotor with a thin elastomeric layer is generally referred to as thin wall or even wall design.
An elastomeric lined stator with a uniform or even thickness elastomeric layer has previously been disclosed in U.S. Pat. No. 3,084,631 on “Helical Gear Pump with Stator Compression”. The prior art has evolved around the principle of injecting an elastomer into a relatively narrow void between the profiled helical bore of a stator and a mandrel with a profiled helical outer surface. The mandrel is then removed after curing of the elastomer and the remaining assembly forms the elastomeric lined stator. The elastomer layer is essentially the last component formed.
The stator bodies mentioned above have a pre-formed profiled helical bore. The profiled helical bore of a stator is generally manufactured by methods such as rolling, swaging, or spray forming, as described in U.S. Pat. No. 6,543,132 on “Methods of Making Mud Motors”, incorporated by reference herein. Similarly, a profiled helical bore can be formed by metal extrusion, as described in U.S. Pat. No. 6,568,076 on “Internally Profiled Stator Tube”, incorporated by reference herein. Further, various hot or cold metal forming techniques, such as pilgering, flow forming, or hydraulic forming, as described in P.C.T. Pub. No. WO 2004/036043 A1 on “Stators of a Moineau-Pump”, incorporated by reference herein, can be used to form a stator with a profiled helical bore.
A stator can also be formed by creating a profiled helical bore in relatively thin metal tubing. This formed metal tube can then be used as the stator by itself or be inserted into a second body with a circular longitudinal bore to form the stator. A stator with a profiled helical bore can also be formed through other process such as sintering or hot isostatic pressing of powdered materials, for example, a metal, or the profiled helical bore can be machined directly into a body.
The prior art designs lead to several inherent manufacturing problems when lining the profiled helical bore of the stator with an injected or molded elastomeric layer, for example, rotational and lateral misalignment. Rotational misalignment can occur when the apex of a lobe of a stator and the apex of an adjacent lobe of the mandrel are not substantially aligned relative to a radial line extending from the central axis during the elastomer injection step. The result is a loss of control of the elastomer thickness on both sides of a lobe. One side of each lobe has an elastomeric layer thicker than intended, and the other side of each lobe has an elastomeric layer thinner than intended.
Another obstacle to forming an elastomeric layer in a stator can be lateral misalignment of the mandrel and the stator. When forming an elastomeric layer, there can be lateral misalignment of the profiled helical bore of the stator and the mandrel. For example, in a long stator there can be lateral misalignment at the mid section even when the ends of the stator and the mandrel are aligned properly due to a sagging of the mandrel and/or the stator. Lateral misalignment during the elastomer injection step creates a loss of control of the elastomer thickness in the profiled helical bore, where one side of the bore has an elastomeric layer thicker than intended and the other side of the bore has an elastomeric layer thinner than intended.
Traditionally, rotors are made of non-compliant material, for example, metal, and the stators are made of non-compliant material housings with an elastomeric lining on the profiled helical bore to run against the rotor. A rotor can be a non-compliant core with a profiled helical outer surface. The core, or bar, can optionally have a bore along the axis for flow bypass. A rotor, or stator, can also be a shell type, such as those rotors available under the registered mark of Even Wall produced by Wilhelm Kächele as shown in prior art FIG. 1. A stator can be metallic tube with a longitudinal bore that is either straight or has a profiled helical form. Straight (e.g., not profiled helical) longitudinal bores can be internally lined with elastomeric material to form the stator profile. A profiled helical bore of a metallic tube is typically for use with thin elastomeric layers.
As the power section of a progressive apparatus, which includes the profiled helical outer surface of a rotor and the profiled helical bore of a stator, is subject to wear and tear, it can be desirable to replace or repair the active surface, i.e., those surfaces of the power section that are exposed to motive fluid. The typically eccentric motion between rotor and stator can create heat that degrades these active surfaces. A resilient material, for example, elastomer, can reach its limit in tensile strength and the high shear and tensile stresses imposed by the eccentrically spinning rotor can tear through any embrittled sections and cause failure of the resilient material. The loss of sections of elastomer is a phenomenon known as chunking and can destroy the usefulness of a progressive cavity apparatus.
A replaceable skin on a rotor and/or in a stator can have many benefits. For example, 1) a skin can be replaced during part refurbishment instead of requiring the entire component (e.g. stator or rotor) to be replaced, 2) rotors and/or stators can be refurbished at a service shop instead of at a central vendor location, 3) smooth continuous skins can be placed over rough and/or discontinuous components, and 4) skins of different thickness can be used to fit the application requirements and/or manufacturing processes.